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INDIAN JOURNAL OF FORESTRY Vol. 23(2): 178·1 !l5. 2000
• FLOWERING PHENOLOGY AND .FLORAL SUCCESS IN MONOCARPIC • BARLERIA INVOLUCRATA VAR. ELATA
Rani M. Krishnan* Salim Ali School ofEcology, Department of Ecology and Environmental Sciences, Pondicherry University, Pondicherry- 605 014. India.
Abstract
The llowering phenology and floral success or the monocarpic understorey shrub species Barleriu illl'olucrtl/a var . .:lata (Acanthaceae) in a medium elevation evergreen forest in ~outhern Western Ghats was examined. No proximate environmental cues could be identified in this study for flowering initiation. Peak flowering was observed in December. Floral predation was low and predators tracked floral availability. Early flowering Individuals suffered greater lloml predation. Floral success was influenced by floral display. visitntion rates and pollen load deposited on stigmas. Wiih the or1sct of synchronous flowering, decreased floral predation and increased fruit set was observed. Plant size influenced the relative reproductive succes.~ of individuals.
Introduction plants continue to be a mystery, although environmental cues (Auguspurger. 1982) or to Tropical species display a variety of flowering resource availability (Harper & Ogden. 1970; Werner, and fruiting patterns (Snow, 1965; Janzen, 1967; 1975; DeJong et al., 1986; Mooney & Chiaricllo, • Foster, 1982; Frankie eta/., 1974; Opler eta!.. 1976; 1984) are implied the list is not exhaustive. Timing Jackson, 1978; Croat, 1978; Putz, 1979; Heideman, of flowering is thus crucial for maximizing 1989). Despite these variations, there is certain reproductive gains especially in monocarpic plants predictability about the flowering and fruiting which have a short, sharp t1owcring window. In episodes of the species (Snow, 1965; Janzen, 1967; some species, synchronous flowering is known to Auguspurger, 1982). Most understorey shrub increase the fruit set by attracting pollinators species in the southern Western Ghats exhibit a (Auguspurger, 1980) and by predator satiation seasonal flowering (Krishnan, 1996). The only (Auguspurger, 1981 ). In contrast, individuals of exception to this rule is the monocarpic genera Polemonium folisissimum increased their Ni/giriamltus which is known to flower reproductive success under low t1oral density synchronously (Duthie. 1890; Robinson, 1935; (Zimmerman, 1980). The relative importance of Steenis, 1938; Hutton, 1949; Santapau, 1950, 1952; predators and pollinators determines the Matthew, 1959). reproductive success of a species and cannot be generalized. Several ecological advantages have been associated with the monocarpic life history strategy. In this paper. the results of phenological Significant of these nrc the selection for outcrossing observations on Barleria invo/ucrata var. e/aw ns the selective tbrce (Whitehead, 1969; Oplcr eta/., (Acanthaccac), a monocarpic undcrstorey shrub, arc 1976). pollinator energetics (Heinrcich & Raven. presented. This study has the following specific 1972) and pollinator attraction and seed predator objectives ( 1) to identify the significance of avoidance (Beattie eta/., 1973; Auguspurgcr, 1981 ). environmental cues for initiation of flowering, (2) to The timing for onset of reproduction in monocarpic examine the impact of floral predation on overall fruit
'Present address: 6309, IJunbar Street. Vancouver BC CQnadu V6NIXJ. Phone : 604 267 1301 Email : mkrani(!i'hounail.com INDIAN JOURNAL OF FORESTRY 179 set. (3) to establish the factors that determine the Barleria flowers are blue and tubular (2. 76 em long). floral success, and (4) to consider the effect of plant with 2 epipetalous functional anthers and 2 size on reproductive success. staminodes. The single ovary has 3 to 6 ovules. The 2-day flowers are protogynous. offering copious Study Site nectar and pollen. The major pollinators arc Apis dorsata and Apis cerena. although Xylncopa bees Kakkachi ( 1400 m), the study site is located in and large butterflies also visit them (Personal the southern end of Kalakad-Mundanthurai Tiger observation). Small birds like rose finch ( Carodacm Sanctuary in the southern Western Ghats in Tamil erytltrifms) and sun birds (Nectarinia asiatica) rob Nadu, South India. The medium elevation evergreen the flower of nectar by piercing the corolla tube forests arc floristically classified as Cu/lenia vertically. The capsules dehisce mechanically in the exarillpta - Mesua ferrea - Palaquium ellipticum - dry season from April. Ghaa travancorica - Diccusocarpus wallichianus type (Ramesh et a/., 1999). The common canopy R/am census and size: Individuals (n"' 36) marked trees include- Caloplryllum austro - indicum. for study were located along a 500 m x I0 m transect. Diccusocatpus wallichianus, Cullenia exarlllata. Monthly census was.taken to record the number of and Pa/aquium ellipticum. The common open and predated flowers, inflorescence, and fruits understorey and sub-canopy trees include for the marked individuals. Every 3 months height Myristica dactyloides, Artocarpus heterophyllus and number of growing points were recorded for and Agrostistachys boomensis. Saprosma individuals. Growing points arc the number uf cmymbosum. Diotacanthus grandis, Agrostistachys meristems available for fm1her growth. Based on the indica, Sarcandra chloranthoides and many number of growing points, plants were classified into species of Nilgiriamlzus arc found in the shrub layer. 2 groups: Small individuals ( 1-30) an,1 large Barleria involucra/a var. e/ata is a monocarpic individuals (> 30). The degree of flowering and shrub species endemic to the evergreen forests of fruiting overlaps for the population was estimated Western Ghats (Gamble, 1967: Henry eta/., 1987), with the number of open flowers recorded monthly und was distributed in a clumped fashion in the following Ludwig and Reynolds ( 1988 ). study area (Krishnan, 1996). Rainfall occurs in 2 peaks following the northeast and the southwest Flowering stages : To characterize flowering stages monsoon. The avcmge annual rainfall is around 3000 of the population, the flowering times were classified mm, with 4 to 5 dry months (data from Nalmukku, as- early (attained when 10%, individuals were in approximately 2 km from the study site). The flower), middle (reached when 50%, individuals were environmental factors considered were monthly in flower) and late (when 75o;,, individuals were in readings of mean maximum and minimum flower). The individual flowering peaks. defined as temperature, rainfall, soil moisture and number of the month in which median number of flowers were bright days. The study was conducted between open, were assigned using the month in which the January, 1990 to June, 1992. median flower opened. Individuals were further grouped into synchronous. pre-synchronous and Material and Methods · post-synchronous flowering individuals based on the time when the median flower opened. During ti1L' Natural histOIJ' : Barleria involucra/a var. elata. synchronous flowering period, maximum number of is a monocarpic understorey shrub growing to a individuals displayed their median flowers. The maximum height of 4 m in the evergreen forests of period before and allcr this was categorized as pre the Western Ghats. Genus Barleria is less known synchronous and post-synchronous flowering than the genus Nilgiriamhus for its synchronous period respectively. flowering behaviour. The species probably flowers synchronously once in 4 or 5 years, although the Floml success : Floral success if defined as the flowering periodicity has not been recorded. number of t1owers successfully initiating fruits. A 180 RANI M. KRISHNAN stepwise regression was used to estimate the Results relative importance of various parameters influencing the overall flora success. The parameters used to Flowering and environmental factors • understand the floral success were, number of Observations from January, 1990 to July, 1991 !lowers and fruits, pollen availability and visitation showed that plant growth was only vegetative. .. rates. To estimate the relationship that explains the Initiation of flowering was negatively correlated with floral success during each phase of flowering, temperature (mean minimum temperature r= -0.664. attributes like mean number of flowers, fn1its, leaves P<0.05; mean maximum temperature r= -0.559, and inflorescence were subjected to non-parametric P<0.05), but did not correlate with rainfall, soil correlation. moisture or number of bright days. Fruiting was mildly aided by soil moisture (r= 0.43 I, P<0.05) and The visitation rates were estimated throughout the number of bright days (r= 0.514, n"" 9, P<0.05). the flowering period, by observing and recording Of the 36 individuals marked for the study, 7 (20%) the number of pollinator visits of the inflorescence. failed to flower and one individual that flowered The observations were spread throughout the failed to fruit. The individuals that failed to flower flowering season in to-minute sessions totalling up were predated by the large mammal herbivores in to 18 hours. The visitors were broadly identified as the dry season. bees, butterflies and birds, and their foraging behaviour for nectar and pollen observed. Total of Flowering stages and fruiting : Flowering 50 anthers from the whole flowering period was progressed slowly from August and peaked in crushed and stained with acetocarmine to count the December. During this period 28% of the total number of pollen grains. Pollen deposition per flowers were presented. Flowering intensified after stigma was estimated by collecting the shriveled December, and by March 84% of the total flowers .. stigmas of 40 flowers for the entire flowering season. were open, Once flowering was initiated, the The stigmas were stained with acetocarmine and ·the participation of individuals increased by I 00% from number of pollen grains counted under a stereo September to October (Fig. I). It required only 2 microscope. months to attain mid-flowering phase, while S
90 l 80 70 60 M ;;.!!. •e so ').·· i'l . . <.) 40 . '. ... 30 "" E •• 20 ~-·· 10 . 0 A s 0 n d J f m a Months
Flg.l: Flowering progression in Barler/a. E Early. lit Mid and L Late flowering phase. The months start with August and end whh April. The flowering phases are defined based on the number of individuals in flower. Early flowering phase is considered when at least I 0% of the individ11als are in 11ower, 50% of the individuals flowered during the mid-flowering phase and 75% of the individuals flowered irt the late nowering phase. INDIAN JOURNAL OF FORESTRY I K I months were necessary for the late flowering phase 8000 to set in. The general flowering overlap value was 0.83. The values obtained for early flowering phase 7000 and mid flowering phase were 0.72 and 0.85 6000 respectively. t 5000 Fruit set on the other hand, was skewed to the ~4000e right. beginning abmptly in October, reaching a peak ...c in February. Fruiting ended in April with the onset ,<;z ~000 of dry season. Early flowers were not very successful in setting fruits. High degree of success ~000 was observed in the late flowering phase when 72% 1000 of individuals set fruit (Fig. 2). Maximum floral success (indicated by fruit to flower ratio) was 0 attained during mid-flowering phase, when maximum A o n d f m a number of individuals flowered in synchrony. The Months general fruiting overlap was 0.85 for the entire f.'ig.3: Floral herbivory in Barleria. population. The fruiting overlap for the middle and The proportion of open nowers prcdah:d w
Fig.2: fruit set in Barlcria. Variables Reg. co-eft'. F I' partial r1 % fr. indicates the percentage of total fruits produced, The values indicated along the % fruit axis are the percentage Pollen 20.11 14.84 0.011 1).75 of individuals in fruit. Fr: 0 ratio indicates the fruit to flower grai11istigma ratio. Open !lowers 0.44 15.59 0.010 0.76
Floral predation : The mass flowering of Barleria Visits -43.25 7,60 0.039 O.(iU attracted lepidopteran caterpillars. Predation of The results of the step-wise regression ;Jrc tabled dq•icting individual floral parts or the entire flower was the variables that separate out in each step. The r'~ 0.93 observed throughout the flowering season. The is signlllcanl to consider the model. Other models did not total number of flowers predated was only 2% of uccount for muny variables us most of them were insignificant to be included in the st11:.:cs~i' c steps. the !lowers produced. The predators seem to track Dependant variable: Fruits. the number of open flowers, until the beginning of synchronous flowering period, after which a drop Pollen availability differed during the lluwering in herbivory is observed (Fig. 3). Fntit predation was phases (F= 15.07, P<0.05), while the pollinator very rare. visitation rate (F= 1.06, ns) and the number of pollen ..,.. ..r
182 RA)';I M. KRISHNA!\ grain deposited per stigma (F= 0.99, ns) did not duration's were compared, the pre-synchronous • change during the different flowering phases. individuals were found to flower briefly (mean= 4.00 months) while the synchronous and post • In the mid-flowering phase, there was a strong synchronous individuals flowered longer (mean "" correlation between pollinator visits and pollen 6.25 & 6.36 months respectively). The mean numher " grains deposited (r= 0.91, P<0.05). For the late of flowers and fruits produced were significantly flowering phase, a correlation between floral display different between the pre-synchronous and post· and number of pollen grains deposited (r= 0.98, synchronous flowering individuals (D= 0.11. X:"' P<0.05) was observed. No significant correlation was 24.99, P ll Table-3: Comparison of traits and relative success flowering observed for the understorey shrub of the small and large plants Hybamhus prrmifo/ius. where the flowering peak is attained in 13 days following the summer rains Character Small (n"' Ill) Large (n: 16) (Auguspurger, 1982). Flowering progression was (mcan±Std) (mcan±Std) characterized by sudden, synchronous onset of Number growing points• I 1.00±8.99 43.00±14.73 flowering by many individuals from the early Number mature leaves• 8.5±7.46 IS.2S:i-5.18 flowering phase onward. This produces a skewed flowering distribution in population. When flowering Plant height (m)• 1.38±0.056 1.96±0.43 is skewed to right. it increases the delectability of Number of flowers the species and attractiveness to pollinators pcrplam• 431.119*366.85 869,38±~ 18.72 (Thomson, 1980). This synchronous onset of Dumtion of flowering flowering also ensures that floral predators arc (months) 5.44.<2.55 5.00±3.03 satiated. These results are similar to the Flowering overlap 0.70 0.84 observations made on Hybanthus prunijo/ius. where Number of fruits per plant• 313.00±JIO.SS 551 .00±431!.01 synchronous flowering reduced the damage by floral and seed predators (Auguspurger, 1981). Fruiting overlap 0.87 0.84 Predator satiation in Barleria is not crucial once the Fruit : flower 0.72 0.63 synchronous flowering phase sets in. Increased floral density due to synchronous flowering ensures The small and large plants were compared using Hcst. When significantly different at P I 184 RANI M. KRISHNA!\ into floral success. It is possible that there is Beattie, A.J.: Breedlove, D.t. & Ehrlich, P.R. ( 1973). • reallocation of resources during the synchronous The ecology if the pollinators and prcu;ttors or Fn1.~era specio.va. F.cology. S4: K 1-91 flowering phase towards maternal function to ensure .. that there is maximum seed set as indicated by the Borchert, R. ( 1983 ). Phenology and <:ontrol or llowcring ,. differences in number of pollen grains per anther io tropic:il trees. Hiotmpica, 15: 81-S'l over the flowering period (Liyod, 1980). Since large Croat, T.B. ( 1978). Flora a{ HatTo Colorado /.1/cmd. floral display ensures that the pollinator visitation Stanford University Press. Stal\l"urtl. California. t;S,\. is almost constant, floral success is ensured. Findings from other studies on mass flowering De Jong, T.; Klinkh11mer, (i.L & l'rlns, A.H. ( 1986). Flowering behaviour of the monocarpk perennial plants suggest that mass flowering endows the Cvnoglossum o.ffkina/1;' L, Nt•••· Phrtologi.w. 103: 219· individuals of a species with 2 major advantages: 229. first, it facilitates highest outcrossing rates Duthie, J.F. ( 1890). 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